The deadly 2010 San Bruno blast was the result of systematic issues with the oversight of gas pipelines.

WASHINGTON — Three powerful accidents in recent years, including a deadly blast in San Bruno, highlight weaknesses in the oversight of how natural-gas providers maintain the largest pipelines in their networks, accident investigators said Tuesday as they issued more than two dozen safety recommendations.

A major effort a decade ago by the federal government to check a rise in violent pipeline failures in "high-
consequence" areas where people are more likely to be hurt or buildings destroyed has resulted in a slight leveling off of such incidents, but no decline, the National Transportation Safety Board said.

And while the frequency of such accidents remains low, they are still more likely to occur in more densely populated areas despite increased safety efforts in those areas, the report found.

The explosion of a PG&E natural-gas pipeline in San Bruno is one accident since 2010 that illustrates many of the systemic problems, the board said. A massive section of pipeline was blown out of the ground in the Sept. 9, 2010, explosion, which killed eight people, injured dozens of others and sparked a fireball that laid waste to 38 homes in the bedroom community that still bears scars from the accident.

The blast was so powerful that residents initially thought there had been an earthquake or jet crash. Because there were no automatic or remotely controlled valves, more than an hour passed before the gas could be shut off.

California Public Utilities Commission judges have recommended a $1.4 billion penalty against PG&E in connection with the blast, which the utility said it is appealing.

A steady increase in pipeline explosions and fires in the 10 years prior to 2003 prompted the Pipeline and Hazardous Materials Safety Administration to adopt safety standards in 2004 for inspecting and maintaining the physical integrity of pipelines, with priority given to high consequence areas.

Since then, state-regulated pipelines — those that don't cross state borders — have had a 27 percent higher incident rate than federally regulated pipelines that traverse more than one state, the report said.

From 2010 to 2013, incidents were overrepresented in high-consequence-area pipelines compared to less-developed areas where the risk to people and property is less, the board said.

The board issued 28 recommendations as the result of the report, most of them to federal regulators. They urged states to adopt more costly pipeline inspection methods that are more likely to find problems. They also urged federal inspectors to work more closely with state inspectors and establish a mentoring program for them.

The board also urged improvements to a national pipeline mapping system so that states and operators could better determine which areas should be designated high consequence and therefore given more attention.